Add an arrow indicator when a tool_button button has a menu
Add an arrow indicator when a tool_button button has a menu

Fixes #67

DeskboxBrazil/Cura,fxtentacle/Cura,ynotstartups/Wanhao,totalretribution/Cura,lo0ol/Ultimaker-Cura,bq/Ultimaker-Cura,totalretribution/Cura,markwal/Cura,ynotstartups/Wanhao,bq/Ultimaker-Cura,hmflash/Cura,senttech/Cura,markwal/Cura,hmflash/Cura,ad1217/Cura,Curahelper/Cura,DeskboxBrazil/Cura,fieldOfView/Cura,fieldOfView/Cura,senttech/Cura,Curahelper/Cura,fxtentacle/Cura,ad1217/Cura,lo0ol/Ultimaker-Cura